emission lights not working as needed in cycles or eevee
Eevee isn't a raytracer like Cycles - lights, reflections, shadows, etc. are effectively 'faked' in order to sacrifice quality for speed. This means that you can get lightening fast renders but does require some 'pre-setup' in order to get the desired results - by way of light probes.

Mesh lights (ie, Emission shaders) produce indirect light and by default Eevee doesn't handle indirect light at all.

For mesh lights you need to use an Irradiance Volume light probe. The Irradiance Volume is a cuboid which encases an area of your scene. It is not rendered itself but Eevee can 'bake' the contents to measure the irradience (emission) within that section of your scene, applying it to any other parts of your scene within that volume.

Within the Render properties in the right-hand properties panel is an Indirect Lighting section and within that a Bake Indirect Lighting button. Clicking this tells Eevee to analyse the indirect lighting (Irradiance Volumes and other light probes) which it can then use to render the scene.